The present invention concerns the general field of power steering systems, and in particular of electric power steering systems, as well as the methods for managing such power steering systems.
More particularly, it concerns the methods which allow determining the absolute position of a movable member of the steering system, for example the absolute angular position of the steering wheel, or the absolute position of the steering rack.
Indeed, this information on the absolute position of the steering wheel (or of the rack) may be necessary for carrying out various onboard functions, such as the automatic return of the steering to the midpoint, the control of the direction change indicators or of directional lights orientation, the parking assist, etc.
To this end, it is known in particular to deduce the absolute angular position of the steering wheel from, on the one hand, an absolute “mechanical” origin position, defined by factory calibration and, on the other hand, a measurement of the relative angular position of the shaft of the steering assist motor, said relative angular position measurement allowing to quantify the angular displacement of the steering wheel, and more generally the displacement of the different constitutive members of the kinematic linkage of the steering, relative to said origin position.
Advantageously, the absolute original position may correspond to the midpoint of the steering system, that is to say to the configuration in which the steering wheel (respectively the rack) is centered, that is to say it is oriented neither to the left, nor to the right.
Nonetheless, in the case of interruption of the electric power supply of the calculator which stores the information related to the original position, for example when replacing the battery, or still subsequently to a mechanical shift occurring between the assist motor and the rest of the kinematic linkage, for example during a skip of the drive belt which links the shaft of said assist motor to the pinion which meshes on the steering rack, the reference frame based on said absolute original position may be lost or distorted.
This is why it is possible to provide for means, integrated to the power steering, which allow recovering said original position.
To this end, it is known in particular to set up, within the power steering, and more particularly at the steering column which supports the steering wheel, an index-type sensor.
Preferably, such index is designed to mark one single reference position (index position) in the same complete revolution of the steering wheel, and more particularly, to generate a pulse when the steering column passes by a predetermined indexed angular position, which thereby constitutes an absolute mechanical reference.
Starting from the information provided by this index, it is possible, during an initial factory calibration operation (learning), to measure the deviation existing between, on the one hand, the reference angular position, which typically corresponds to the angular position occupied by the steering wheel when crossing the index and, on the other hand, the position of the selected absolute origin, which typically corresponds to the angular position occupied by this same steering wheel when the steering is in its midpoint.
In particular, the angular positions may be measured by means of any suitable sensor, and for example by means of an angular position sensor associated to the shaft of the assist motor.
Ultimately, the absolute origin of the steering mechanism may be defined as being the (algebraic) sum of the reference position (position tracked by the index) and of an offset-type corrective term, which corresponds to the aforementioned deviation, initially assessed by calibration.
According to this principle, it is possible at any moment, as long as the information on the reference position (index position) is known, to recover the absolute origin of the reference frame of the steering, by adding to this reference position (index position) a corrective term, which corresponds to the angular deviation (offset) initially measured during the factory calibration operation.
In practice, by combining the information, on the one hand, derived from the index and, on the other hand, from the sensor which measures the relative angular position of the shaft of the assist motor, and more particularly by identifying the relative angular position of the motor shaft at which the index pulse is detected, it is advantageously possible to re-calibrate the measurement reference frame, whenever necessary, by recovering its absolute origin.
In this manner, it is possible to assimilate again, in this re-calibrated reference frame, the relative angular position of the motor shaft to the absolute angular position of the steering wheel.